The use of so-called "single-site" catalysts such as metallocene catalysts to prepare polyethylene having a narrow molecular weight distribution is well known. The "linear low density polyethylene" (or "LLDPE", a copolymer of ethylene and a higher alpha olefin) prepared with such catalysts typically exhibits a very uniform composition distribution (i.e. the comonomer is very uniformly distributed within the polymer chains). The combination of narrow molecular weight distribution and uniform composition distribution distinguishes these polymers from "conventional" LLDPE which is commercially manufactured with a Ziegler Natta catalyst or a chromium catalyst. In particular, the conventional LLDPE products have a broader molecular weight distribution and a broader composition distribution. These compositional differences are manifested in the form of differences in the physical properties of the two types of LLDPE polymers. Most notably, LLDPE prepared with a single site catalyst has improved impact strength and optical properties in comparison to "conventional" LLDPE. However, one advantage of the "conventional" LLDPE is that it is usually easier to "process" in its existing mixers and extruders. Accordingly, it would be highly desirable to prepare LLDPE products which possess the improved physical properties offered by single site catalysts and retain the broad molecular weight distribution (for improved processability) which is associated with conventional LLDPE.
One approach which has been used to achieve this object is the use of mixed catalyst systems. For example, U.S. Pat. No. (USP) 4,530,914 (Ewen et al, to Exxon) teaches the use of two different metallocenes and U.S. Pat. No. 4,701,432 (Welborn, to Exxon) teaches the use of a supported catalyst prepared with a metallocene catalyst and a Ziegler Natta catalyst. Many others have subsequently attempted to use similar mixed catalyst systems as may be quickly ascertained by reviewing the patent literature.
However, the use of "mixed" catalyst systems is often associated with operability problems. For example, the use of two catalysts on a single support (as taught by Welborn in U.S. Pat. No. 4,701,432) may be associated with a reduced degree of process control flexibility (e.g. If the polymerization reaction is not proceeding as desired when using such a catalyst system, it is difficult to establish which corrective action should be taken as the corrective action will typically have a different effect on each of the two different catalyst components). Moreover, the two different catalyst/cocatalyst systems may interfere with one another--for example, the organoaluminum component which is often used in Ziegler Natta or chromium catalyst systems may "poison" a metallocene catalyst. Accordingly, a "mixed catalyst" process which mitigates some of these difficulties would be a useful addition to the art.